Microfluidic devices and methods are of significant and increasing importance in biomedical and pharmaceutical research. Microfluidic technology is applied to sequentially synthesize or batch synthesize fine chemicals and pharmaceuticals. Continuous flow micro-reactors have also been used instead of batch micro-reactors to synthesize fine chemicals and pharmaceuticals. Continuous flow micro-reactors have been demonstrated for chemical processes on the nanoliter to liter scales, with advantages of enhanced heat transfer performance, faster diffusion times and reaction kinetics, and improved reaction product selectivity.
A typical microfluidic device includes micro-channels or other microfluidic features with characteristic cross-section dimensions in the range of microns to 100's of microns. In such channels or features, viscous forces, surface tension, and applied external forces generally dominate over gravitational forces. Such a regime of operation makes it difficult to perform some conventional macro processes in the traditional manner, where gravity is utilized. Conventional processes that utilize gravity include bubble extraction into a headspace, boiling, liquid phase extraction, and sedimentation.
A chamber large enough to utilize gravitational forces on the fluid can be created in a planar format, consistent with a typical microfluidic credit card device format. However in constraining the chamber to a planar format, new factors must be considered. These factors include maximizing the free surface for processes that require a free surface, such as evaporation, and controlling the wetting or wicking of fluid on the chamber walls. When capillary forces and surface wetting, drive liquids excessively up or along the walls in the chamber, unintended wetting of certain areas of the chamber occurs. When the liquid having one or more compounds is transported by wetting inside the chamber, a side effect is that species in the liquid can reach areas of the microstructure that was not intended. If the species adhere to surfaces in these regions, this can lead to unwanted adhesion and losses. In some cases, such as corners, it can be difficult to release the adhered species.